1. Field of the Invention
This invention relates to a plasma addressed display device including a flat panel having a display cell and a plasma cell, that one overlapped, and a peripheral area. More particularly, it relates to a technique for achieving high resolution of scanning lines formed in the plasma cell.
2. Description of the Related Art
The structure of a plasma addressed display device, disclosed in, for example, the Japanese Laying-Open Patent H-4-265931, is shown in FIG. 1. As shown therein, the plasma addressed display device is of a flat panel structure comprised of a display cell 1, a plasma cell 2 and a common intermediate sheet 3 interposed therebetween. The intermediate sheet 3 is formed by an ultra-thin glass plate, termed a micro-sheet. The plasma cell 2 is made up of a lower glass substrate 4, connected to the intermediate sheet 3, and a dischargeable gas is sealed in a gap defined therebetween. On the inner surface of the lower glass substrate 4 are formed stripe-shaped scanning electrodes operating as anodes A and cathodes K arranged as sets. Plural barrier ribs 7 are provided for demarcating the sets of the anodes A and the cathodes K from one another. The gap charged with the dischargeable gas is split by these barrier ribs to delimit discharge channels 5. The neighboring discharge channels 5 are isolated from one another by the barrier ribs 7. These barrier ribs 7 can be printed by the screen printing method, with the top sides of the barrier ribs compressing against the sides of the intermediate sheet 3. Within the discharge channels 5, surrounded by the paired barrier ribs 7, plasma discharge is induced between the anodes A and the cathodes K. The intermediate sheet 3 and the lower glass substrate 4 are interconnected by e.g., glass frit.
The display cell 1 is constituted by a transparent upper glass substrate 8, which is connected to the other surface of the intermediate sheet 3 by a sealant to define a gap. Within this gap is sealed a liquid crystal 9 as an electro-optical material. A signal electrode Y is formed on the inner surface of the upper glass substrate 8. In each intersecting point of the signal electrode Y and the discharge channel 5 is formed a pixel to form a matrix of pixels. On the inner surface of the glass substrate 8, there is provided a color filter 13 to allocate three prime colors R, G and B to each pixel. The flat panel, constructed in this manner, is of a transmission type. For example, the plasma cell 2 and the display cell 1 are arranged on the incident side and on the outgoing side, respectively. On the plasma cell side is mounted a backlight 12.
With the above-described plasma addressed display device, the row-shaped discharge channels 5 in which occurs plasma discharge, are switched and scanned line-sequentially, while picture data are applied to column-shaped signal electrodes Y on the display cell side in synchronism with the scanning to effect display driving. If plasma discharge occurs in the discharge channels 5, the inside of the discharge channel 5 is at an anode potential substantially uniformly to effect pixel selection on the scanning line basis. That is, each discharge channel 5 corresponds to a scanning line and operates as a sampling switch. If, with the plasma sampling switch on, pixel data is applied to each pixel, sampling takes place to control the pixel turning on or off. With the sampling switch turned off, the pixel data are held in the pixels. That is, in the display cell 1, the incident light from the backlight 12 is modulated into outgoing light, depending on the picture data, to display a picture.
FIG. 2 shows only two pixels 11. For assisting in the understanding, only two signals electrodes Y1, Y2, a sole cathode K1 and a sole anode A1 are shown. Each pixel 11 has a layered structure of the signals electrodes Y1, Y2, a liquid crystal 9, an intermediate sheet 3 and a discharge channel 5. The discharge channel is connected substantially to the anode potential during plasma discharge. If, in this state, picture data is applied across the signal electrodes Y1, Y2, electrical charges are implanted into the liquid crystal 9 and the intermediate sheet 3. On termination of the plasma discharge, the discharge channel is restored to the insulated state, so that the potential is the floating potential such that the implanted electrical charges are held in the respective pixels by way of effecting the sample-and-hold operation. Since the discharge channel operates as a sampling switch element provided in each pixel, it is depicted symbolically by a switching symbol S1. On the other hand, the liquid crystal 9 and the intermediate sheet 3, held between the signal electrodes Y1, Y2 and the discharge channel, operate as sampling capacitors. If, by line-sequential scanning, the sampling switch S1 is in a conducting state, picture data is written in the sampling capacitors, such that the respective pixels are turned on or off depending on the data voltage level. After the sampling switch S1 is in the non-conducting state, the data voltage is held in the sampling capacitor to effect the active matrix operation of the display device. Meanwhile, the effective voltage applied to the liquid crystal 9 is decided by capacity division with respect to the intermediate sheet 3.
In the above-described plasma addressed display device, if the picture is to be improved in resolution, the pixels arranged in a matrix configuration need to be increased in pixel density. For reducing the pixel size in the horizontal direction, that is in the row direction, it suffices to reduce the line width of the column-shaped signal electrodes. On the other hand, for reducing the pixel size in the vertical direction, that is in the column direction, it suffices to reduce the arraying pitch of the row-shaped discharge channel. However, the respective discharge channels are isolated from one another by the barrier ribs. Due to limitations in the machining techniques, it is difficult to reduce the thickness of the barrier ribs drastically, such that there is set a minimum thickness for assuring e.g., mechanical strength. Therefore, if the arraying pitch of the discharge channels is diminished, the area taken up by the thickness of the barrier ribs is relatively increased to diminish the area of the opening through which is transmitted the light. Stated differently, the larger the number of the discharge channels, that is the scanning lines, the lower is the open area ratio in the panel. Moreover, since the barrier ribs are of a certain height, these obstruct the obliquely incident light rays. Thus, the shorter the arraying pitch of the barrier ribs, the larger is the ratio of obstruction of the obliquely incident light, thus leading to the narrow viewing angle from the viewer.
If it is attempted to reduce the arraying pitch in the plasma addressed display device, the open area ratio is necessarily reduced due to limitations in the manufacturing process of the barrier ribs or scanning electrodes. The result is insufficient brightness of the display. If, for compensation, the light emitting volume of the backlight is increased, the power consumption is increased. If the barrier ribs or the electrode structures are reduced in size, the rate of occurrence of defects is necessarily increased, thus giving rise to the incompatibility between the productivity and the opening area ratio. For example, in the plasma cell structure shown in FIG. 3, the arraying pitch of the discharge channels 5 is 1000 xcexcm, with the width of the barrier ribs 7 being 200 xcexcm and with the width of the anode A or the cathode K being 200 xcexcm. Thus, the open area ratio, of the illustrated panel is 1xe2x88x92(200+200+200)/1000=0.4, or 40%. If the arraying pitch is reduced from 1000 xcexcm to 700 xcexcm, the open area ratio is as low as 1xe2x88x92(200+200+200)/700=0.14 or 14%. In such case, the open area ratio can be raised to a limited extent by reducing the electrode width of the anode A or the cathode K. However, if the electrode width is reduced, the production yield is lowered due to line breakage to lower the productivity significantly.
It is therefore an object of the present invention to overcome the above-mentioned disadvantages in the prior art.
A plasma addressed display device includes a flat panel, a scanning circuit and a signal circuit. The flat panel is of a superimposed structure of a plasma cell having scanning electrodes in a row configuration and a display cell having scanning electrodes in a column configuration. The scanning circuit sequentially applies selection pulses to the scanning electrode to scan the display panel. The signal circuit writes picture data in the signal electrode in synchronism with this scanning. On the plasma cells, there are formed reciprocally isolated discharge channels in a row configuration. To each discharge channel, charged with an electrically dischargeable gas, are allotted plural scanning electrodes. As characteristic of the present invention, the scanning circuit sequentially applies the selection pulses to the scanning electrode allotted to a sole discharge channel to produce electrical discharge to form at least two scanning lines to a sole discharge channel. This realizes a high opening area ratio and high resolution of the plasma addressed display device. The signal circuit writes picture data of the same polarity on two scanning lines, that is the forward scanning line and the backward scanning line, while writing picture data of opposite polarity to the two scanning lines, that is forward and backward scanning lines, belonging to a sole discharge channel, to effect AC driving of the display cell. As similarly characteristic of the present invention, the signal circuit corrects the picture data written in the backward side one of the forward side and backward side scanning lines affected by the polarity switching in meeting with the picture data of the forward side scanning line not affected by the polarity switching. Stated differently, the picture data written in the respective scanning lines in the discharge channel is corrected at a preset correction value to cancel the effect of the polarity switching.
According to the plasma addressed display device of the present invention, at least two scanning lines, namely a forward side scanning line and a backward side scanning line, are allotted in the reciprocally isolated discharge channels. Since the scanning line density is at least twice that of the conventional device, the pixel can be arrayed at a denser pitch. Conversely, should the same pixel density as that of the conventional device suffice, the arraying pitch of the discharge channels can be at least twice that of the conventional device, thus leading to improved productivity and improved open area ratio. Also, according to the present invention, two scanning lines can be formed per discharge channel by allotting two scanning electrodes, for example, per discharge channel. Conversely, with the conventional plasma cell, a scanning electrode made up of an anode and a cathode is allotted to a sole discharge channel to form a sole scanning line. Thus, if the same number of scanning lines are formed in the conventional device and in the present invention, the number of the scanning electrodes can be one-half that of the conventional device to improve productivity and the open area ratio in a similar manner.
In particular, in the present invention, picture data of the positive polarity, for example, are written in the two scanning lines, that is the forward scanning line and the backward scanning line, belonging to the sole discharge channel, while picture data of the negative polarity are written in the two scanning lines, that is the forward scanning line and the backward scanning line, belonging to the next discharge channel, to effect AC driving of the display cell. This AC driving is effective to prolong the service life of the display cell employing the liquid crystal as an electro-optical substance. In this case, the previously selected forward one of the two scanning lines formed in a sole discharge channel is of the same polarity as the subsequently selected scanning line and hence is not affected by polarity switching which is based on the AC driving. Conversely, the subsequently selected scanning line is affected by the next polarity switching. The result is that, if the picture data written in the posterior scanning line is of the same value as the previous scanning line, the luminance value as actually observed becomes different: In order to prevent this, the picture data written in the posterior scanning line affected by polarity switching is previously corrected in meeting with the picture data written in the previous scanning line not affected by polarity switching.
According to the present invention, at least two scanning lines, that is forward and backward scanning lines, are provided in a sole discharge channel in a plasma addressed display device. For forming at least two scanning lines in the sole discharge channel, at least two scanning electrodes are provided in the sole discharge channel. With this structure, the numbers of the scanning electrodes and the barrier ribs can be reduced to one-half those of the conventional device, thus significantly improving the productivity. On the other hand, the open area ratio can be improved to increase the brightness as a display to reduce the power consumption of the backlight correspondingly. In addition, by halving the number of the barrier ribs, it becomes possible to release limitations on the angle of field of view in the up-and-down direction of the viewing plane to increase the angle of field of view. In particular, if the picture data written in the posterior scanning line affected by polarity switching are corrected in meeting with the picture data written in the previous scanning line not affected by polarity switching, at the time of AC driving of the display cell, the picture display quality can be improved significantly.